Some quotes:

"It is a deeper subject than I appreciated and, I begin to suspect, deeper than anyone yet appreciates. To see it whole is certainly a daunting, for the moment even impossible, task." (Robert Langlands, writing about the theory of automorphic forms.)

"Better known for other work." (Lara Pudwell, after referencing a result due to Ted Kaczynski.)

"Understanding what a theorem means is a prerequisite to understanding its proof." (Contemporary Abstract Algebra, pp. 82, Theorem 4.3.)